1. Field of the Invention
The present invention relates to a brushless polyphase DC motor, and particularly to a technique which is effectively applied to a brushless polyphase DC motor and which enables an increase in torque during rotation in a spindle motor for a magnetic disk device.
2. Description of the Related Art
A brushless DC motor, for example, comprising a stator generating a magnetic field in an excited state and a rotor obtaining turning force by the electromagnetic interaction with the stator is frequently used as a spindle motor for a magnetic disk device. The rotation of such a brushless DC motor is controlled by an electronic circuit formed into a semiconductor chip.
For example, in a three-phase coil brushless polyphase DC motor, each of coils is intermittently and alternately charged with electricity in opposite directions by synchronously supplying currents flowing in a normal direction, stopping and flowing in a reverse direction to generate a magnetic flux in each of cores, as shown in EUROPEAN PATENT APPLICATION Publication No. 0495611 A2. In this case, the waveform of the magnetic flux density is substantially the same as that of the current supplied to the coils.
The motor can be driven by attraction and repulsion between the magnetic flux density and a permanent magnet for generating driving force (torque). In this case, the intensity of the torque T has the relational expression, T (torque) .alpha.i (current) . .DELTA.B (change in magnetic flux density) , i . e . , the torque is in proportion to the current and the change in magnetic flux density. The torque is thus increased by increasing the current or the change in magnetic flux density.
However, since the waveform of the magnetic flux density produced in each of the cores is substantially the same as that of the current supplied to each of the coils in a conventional brushless polyphase DC motor, the current must be increased for increasing the torque, thereby causing the problem of increasing the capacity of a power source.
The applicant of this invention has proposed a method of obtaining a large change in the magnetic flux density by double stepping drive in which a current is converted from the normal direction to the reverse direction without a stopping period at the starting of a motor, thereby generating a high torque. However, in this case, since the motor is rotated with a high torque only at the starting thereof, and since currents flowing in the normal direction, stopping and flowing in the reverse direction are supplied during the rotation of the motor, the torque waveform is a general waveform, and thus the torque cannot be increased during the rotation.